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A team of researchers led by Stefan Raunser from the Max Planck Institute of Molecular Physiology in Dortmund has now been able to fully unveil the sophisticated mechanism using the bacterium Photorhabdus luminescens as an example.
Bacterial toxins are among the most effective natural poisons. The strongest toxins include e.g. the tetanus and botulinum toxins (botox), already toxic when only a thousandth of a gram is administered. Bacteria have developed highly varied strategies and mechanisms to introduce their poisons into organisms. The bacterium Photorhabdus luminescens, the plague bacterium Yersinia pestis, as well as germs from the salmonella family, use so-called Tc toxins which consist of several components (TcA, TcB, TcC). Until recently, it was unclear how these subunits work together.
Cases - Bio-molecules - Bacterial - Toxins - Crystallography
In most cases, large bio-molecules such as the complex bacterial toxins cannot be structurally analysed using traditional X-ray crystallography, because they cannot be converted into the required crystalline state. Cryo-electron microscopy, however, does not require the samples to be crystallized. Imaging of even large complexes becomes possible, by freezing them extremely quickly and examining them directly under the microscope at minus 196 degrees. Using cryo-EM the team of Stefan Raunser was able to determine the three-dimensional structure of the Photorhabdus luminescens toxin for the first time in near-atomic detail. The structures showed, that the largest subunit of the poison complex, TcA, resembles a bell, consisting of a channel surrounded by a shell. The upper part of the bell binds the poison capsule formed by the subunits TcB and TcC. Receptors on the cell membrane recognize the lower part of the bell and the loaded poison complex is bound.
Once the pH value of the surrounding medium changes, the outer shell of the toxin opens up, thus revealing the channel. A protein chain kept under high pressure...
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